Mark Zuckerberg realized Google+ could be a killer feature baked into Android, the world's most popular operating system. The more people share on mobile phones through Google+, the bigger the threat Google+ becomes to Facebook.

To fend off the attack, Zuckerberg reached out to Apple:

The two companies have held multiple rounds of discussions, according to people with knowledge of the talks. But they have yet to find a compelling way to collaborate, perhaps because their courtship got off to a rocky start. Last year Facebook rebuffed Apple's attempt to connect Ping, a new social network built around iTunes, with Facebook, purportedly for technical reasons. It was a rare public rebuke for Apple, and Steve Jobs personally called some reporters to voice his displeasure. That Apple chose to bake Twitter, not Facebook, into the most recent version of its mobile operating system has not helped. Still, the two companies continue to talk, knowing full well that an alliance could help them fend off a common enemy.

More details from the Fortune story that show Facebook is treating Google+ as a real threat:

..Facebook employees put in some serious overtime during the summer lockdown. This had happened only once before in recent years at Facebook: After word leaked that Google was starting work on a "Facebook killer" in summer 2010, Zuckerberg called on engineers to work nights and weekends for 60 days to revamp key social features like photos, groups, and events. Just as it did then, the cafeteria opened up on evenings and weekends this summer, and children dropped in for dinners and good-night hugs before their parents logged back on for late nights. By September, Facebook had released a slew of new features like better grouping tools to mirror those Google+ circles. Says one member of the product and engineering team: "[Google] can throw all the money in the world, including hundreds of people, at this. So people were, like, This is serious, and we should take it seriously."